Walls made from gypsum wallboard are conventionally constructed by attaching the wallboard panels to studs, and filling and coating the joints between the panels with a specially formulated composition called a joint compound. The wetted joint compound (taping grade) is placed within the joint formed by the abutting edges of the wallboard panels, and a fiberglass or paper reinforcing tape is embedded with the joint compound which is then permitted to dry. When the joint compound is dry, a second joint compound (topping grade) is applied over the joint, and it too Is permitted to dry. Thereafter, the joint compound may be lightly sanded and the wall may be conventionally finished with a decorative coating (paint or wallpaper); or if necessary to completely hide the joint, there may be a third application of a joint compound, and after it is dry, it may be lightly sanded and a decorative coating applied to the wall. Instead of using two grades of joint compound, an all purpose joint compound is commercially available which is used both for embedding the tape and for the finish coat(s).
Conventionally, all joint compounds contain a filler, a binder and a thickener. The taping grade joint compound generally contains more binder than the topping grade. The conventional fillers are calcium carbonate, calcium sulfate dihydrate (gypsum), and calcium sulfate hemihydrate (plaster of Paris). The calcium sulfate hemihydrate is used only in setting type joint compound as disclosed in U.S. Pat. No. 3,297,601. However, in current construction practices, it is preferred to use a ready mixed or pre-wetted joint compound which contains either a calcium carbonate or gypsum filler.
As disclosed In U.S. Pat. No. 3.891,453, reissued as U.S. Pat. No. Re. 29,753, for many years joint compounds contained asbestos as a key ingredient. Asbestos provided nonleveling, slip and water retention; however, as a result of a health hazard, asbestos was removed from joint compounds many years ago. The most commercially successful substitute for asbestos in joint compounds Is attapulgus clay, as claimed in U.S. Pat. No. Re. 29,753. Attapulgus clay does provide a joint compound having working properties similar to those of joint compounds which contained asbestos, but there are also adverse effects resulting from the use of attapulgus clay. It has been found that attapulgus clay adversely affects crack resistance and the viscosity stability of ready mixed joint compounds. U.S. Pat. No. 4,370,167 discloses the use of sepiolite clay as an asbestos replacement in joint compound properties and it is believed that the sepiolite clay also adversely affects joint compound.
As mentioned above, most commercial joint compound formulations contain a thickener, usually a cellulosic material. Some joint compound formulators dislike the relatively slow hydrating cellulosic thickeners and the high viscosity provided thereby, and some chemical suppliers are recommending liquid thickeners to reduce batch lumping tendencies and the longer mixing times which are associated with dry powder cellulosic thickeners. In-situ or associative thickener emulsions have been recommended as a substitute for cellulosic thickeners in joint compounds. These associative thickeners are supplied as stable latex emulsions which are reactive, acidic thickening agents. The addition of an alkaline agent neutralizes the acidic groups whereby a clear, viscous solution is formed which is an effective thickener for many joint compound formulations.
A further development in joint compounds is disclosed in U.S. Pat. No. 3,386,223 wherein glass (inorganic) microbubbles are incorporated into the composition. The purpose of these glass microbubbles is to provide a joint compound having little or no shrinkage upon the drying of the compound after application to a wall or ceiling surface.
U.S. Pat. No. 4,454,267 discloses a lightweight joint compound for use in finishing joints between gypsum wallboards, comprising a filler, a binder, a specially treated expanded perlite, a non-leveling agent, and a thickener. Attapulgus clay is the non-leveling agent which provides slip and thixotropic properties. The expanded perlite, treated to render it water-insensitive, provides the lightweight property.